then became very faint. I next slipped the wire coil entirely off the magnet, and simply held it touching the pole side-ways. The sounds were still quite audible, and continued to be so when the coil was removed an inch or more away from the magnet. This led me to try the effect of dispensing altogether with the steel magnet, and merely holding the helix of insulated wire opposite the centre of the vibrating plate. With an iron disc a very faint sound was heard, but it became more distinct when a disc of copper was used. From this it would seem, that the vibrations producing the sound are caused by the attraction between the currents in the helix and the induced currents in the copper disc.

Still using the same transmitting instrument, I next employed a receiver, in which a soft iron core, carrying the insulated wire, was rigidly attached to the vibrating plate. This was accomplished by rivetting the head end of a screw-nail into its centre, and winding the wire round it. The pole of a steel magnet was then fixed, just clear of actual contact, opposite the head of the nail, and, with this arrangement, sounds were distinctly audible, though not so loud as when the wire coil surrounded the magnet.

With the view of rendering the telephone an instrument for detecting the existence of very feeble currents, I constructed a pair with magnets similar to those already described, but with ferrotype discs. These were joined together by a strong semicircular spring in such a way that they could be put on, after the manner of spectacles, and stick close to the sides of the head, inclosing both ears. India-rubber rings were provided for the double purpose of excluding all extraneous sound, and avoiding any disagreeable pressure on the ear.

With this arrangement, I proceeded to test for the existence of thermo-electric currents. A copper and iron junction was inserted in the circuit of the telephone, and attached to a spring in such a way that it could be made to vibrate rapidly out and into a gas flame. A distinct grating noise was heard in the telephone. A very peculiar rasping sound was produced when the ends of a copper and iron wire were rubbed together while hot, and also when one of the wires was attached to a file, and the hot end of the other drawn rapidly along it.

4. On the Theory of the Telephone. By

Prof. George Forbes.

The Telephone, invented by Mr Graham Bell, is an instrument by means of which any sounds, musical notes or spoken sentences, sounded into an instrument at the sending end of a telegraphic wire may be reproduced at the receiving end. The theory of the Telephone is two-fold. First, the mechanical theory of the nature of sounds and of speech; and secondly, the theory of the action of the instrument. The first part is well known. All sounds consist of a succession of waves propagated through the air, the rate and intensity of their succession determining the nature of the sound, in pitch, loudness, and tone. If the same succession of waves as a speaker makes in using his voice can be reproduced in the air in contact with the ear of any other person, by any means whatever, the latter person will hear a fac-simile of the sound uttered by the former. A theory of the action of the Telephone is distinctly given by the inventor in the specification of his patent. The receiving and sending instruments are identical. Round the end of a bar magnet a coil of fine wire is wound, connected with a telegraph wire on the one hand and the earth on the other. In front of these a thin iron plate is caused to vibrate by the sounds uttered in its neighbourhood. Consequently, it approaches and recedes from the pole of the magnet. This alters the magnetism in the neighbourhood. In Faraday's language, the lines of force are altered in position, so as to go across the coil of wire.' In consequence of this, by the well-known laws of electromagnetic action, a current of electricity is sent in one direction with each approach of the vibrating plate, and in the opposite direction with each recession. This succession of currents reaches the receiving end with a strength proportioned to the extent of vibration of the iron plate. Here they circulate through the coil, and so intensify or diminish the magnetism of the bar. Thus, the vibrating plate at the receiving end is more or less attracted, according to the direction of the current. Hence, the plate at the receiving end vibrates to and fro in vibrations which are synchronus and proportionate to those at the sending end; and so the sounds are reproduced.

This theory gives a vera causa for the action of the instrument in its normal form, and probably explains in part the true action which takes place, but the author believes that this is not the whole of the action. The fact that the inventor was working upon this theory when he designed the instrument, does not prove the theory to be correct, as is shown by his having at first employed a powerful battery with the instrument, believing it to be necessary, and it was only after diminishing the number of cells without impairing the action of the Telephone that it occurred to him to try it without a battery at all, when he found the action to be equally good.

The author's mistrust of the theory commenced when he learnt that a thick iron plate might be used, whose vibrations must be extremely minute. And when he learnt that the instrument might be used with plates of glass, wood, tinfoil, or vulcanite, or even without any plate at all, he was convinced that the theory was imperfect, and resolved to propose another theory which seemed more consistent with facts.

This theory is divided into two parts-the action at the sending end, and that at the receiving end. It is very simple, and is founded upon two well-known experiments.

The theory of the action at the sending end is founded upon an experiment by Sir William Thomson, who finds that iron subject to magnetic induction has its magnetism increased when slightly stretched in the direction of its magnetization, and we may assume that it is diminished when compressed. Now, what happens in the Telephone? Take the simplest case which has been tried, i.e., where there is no vibrating plate. Sound waves (which are capable of passing through solid bodies as they do through the air) strike the end of the bar magnet, and are propagated through it. Waves of alternate compression and extension pass through the length of the magnet. Consequently, the magnet is alternately increased and diminished in intensity. Hence, currents are generated in the coil of wire, and transmitted through the telegraph wire alternately, in opposite directions, as in the theory hitherto adopted. The presence of a vibrating plate of any kind may help this action, by increasing the intensity of the waves transmitted through the bar magnet; and if the plate be of iron it will undoubtedly help to propagate

these currents.

According to the author's theory, the action at the receiving end is explained by an experiment ascribed to Page. If a bar of iron be surrounded by a coil of wire connected with a battery, a chink is heard when the connection is broken, and a feebler one when the connection is re-made. If the connection be made, and broken rapidly by attaching one end of the wire to a file and drawing the other along the file, a series of chinks are heard succeeding each other at the same intervals as the contacts are made and broken. It is due to the lengthening of the bar on magnetisation and the rearrangement of the molecules of the iron. It takes place also when the iron remains partially magnetised. In this case successive contacts increase the magnetism of the bar. In the Telephone successive currents pass in opposite directions through the coil. the bar is lengthened and shortened alternately, and chinks are made at intervals corresponding to the intervals between successive waves at the sending end. Hence the sound waves at the sending end are reproduced at the receiving end alike in rapidity and in intensity. When a vibrating plate of any kind is used it increases the intensity of the sound, and if it be of iron the attraction will increase the effect.

Hence

Since a chink is given off both when the magnetism is increased and diminished it might be expected that the note should be heard an octave higher. But since the one is more intense than the other, the combined effect should be to give two notes, a loud one in the tone of the speaker, and a feeble one an octave higher. This might account for the common remark that a piano sounds wiry through a Telephone, and that a clear lady's voice is squeaky. But it is not certain that this effect is produced, for the chink is produced at least in part by the lengthening and shortening of the magnet. Either of these alone will give a chink, but when following each other in rapid succession they compress and rarefy the air in contact, in periods exactly agreeing with the compressions and rarefactions originated at the sending end.

In confirmation of the supposition that the sound is produced by successive chinks of the material of the magnet, it was stated that in a very large and massive pair of Telephones made by Mr Wm. Bottomley, jun., when any one is speaking through one of them the sound is distinctly (though not articulately) heard by every one in the

VOL. IX.

4 E

2 memar ftuan IN 2 DEDOSm intressed when sightly srazied in de özeAI IZ BEDCAmin, and we may assume that is a Šmitushed wie sompressed. Now, what happens in the Teleph.me! Take the singuest case which has been tried, La, where there is no rheating plate Sound waves which are capable of passing through solid bodies as they do through the airy strike the end of the bar magnet, and we priparned throgh it. Waves of alternate compression and extension pass through the length of the Consequently, the magnet is alternately increased and diminished in intensity. Hence, currents are generated in the coil wire, and transmitted through the telegraph wire alternately, in

directions, as in the theory hitherto adopted. The presence vibrating plate of any kind may help this action, by increasing intensity of waves transmitted through the bar magnet;

if the plat

iron it will undoubtedly help to propagate